Fuel cell power plant with real and reactive power modes

ABSTRACT

An illustrative example fuel cell power plant includes a cell stack assembly having a plurality of fuel cells configured to generate electricity based on an electrochemical reaction. The power plant includes a capacitor, a plurality of inverters, and at least one controller that is configured to control the plurality of inverters in a first mode and a second mode. The first mode includes the cell stack assembly associated with at least one of the inverters. A cell stack assembly and the associated inverter provide real power to a load external to the fuel cell power plant in the first mode. The second mode includes at least a second one of the inverters associated with the capacitor. The capacitor and the second one of the inverters selectively provide reactive power to or receive reactive power from a grid external to the fuel cell power plant in the second mode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/060,944, which was filed on Mar. 4, 2016.

TECHNICAL FIELD

This disclosure generally relates to fuel cell power plants. Moreparticularly, this disclosure relates to a fuel cell power plant havingreal and reactive power modes.

BACKGROUND

Fuel cells are devices that generate electrical power based on anelectrochemical reaction. Fuel cell power plants are known that includea cell stack assembly having a plurality of fuel cells for generating adesired amount of power.

Typical fuel cell power plants generate real power based on the outputof the cell stack assembly. It is known, for example, to utilize a setof inverters for providing AC power output based on DC power generatedby the cell stack assembly.

To the extent that known fuel cell power plants have reactive power onlycapabilities, those are typically limited to operating as a static VARcompensator.

SUMMARY

Illustrative example embodiments of this invention include a fuel cellpower plant that has the capability of operating in a real power modeand a reactive power mode in which the only power output from the powerplant is reactive power.

An illustrative example fuel cell power plant includes a cell stackassembly having a plurality of fuel cells configured to generateelectricity based on an electrochemical reaction. The power plantincludes a capacitor, a plurality of inverters, and at least onecontroller that is configured to control the plurality of inverters in afirst mode and a second mode. The first mode includes the cell stackassembly associated with at least one of the inverters. The cell stackassembly and the associated inverter provide real power to a loadexternal to the fuel cell power plant in the first mode. The second modeincludes at least a second one of the inverters associated with thecapacitor to selectively provide reactive power to or receive reactivepower from a grid external to the fuel cell power plant.

An illustrative example method of operating a fuel cell power plantincludes controlling a plurality of inverters in a first mode and asecond mode. A cell stack assembly and at least one associated inverteris used for providing real power to a load external to the fuel cellpower plant in the first mode. A capacitor and at least a second one ofthe inverters is used for selectively providing reactive power to orreceiving reactive power from a grid external to the fuel cell powerplant in the second mode.

Various features and advantages of at least one disclosed exampleembodiment will become apparent to those skilled in the art from thefollowing detailed description. The drawings that accompany the detaileddescription can be briefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrated selected portions of a fuel cell powerplant designed according to an embodiment of this invention operating ina first mode.

FIG. 2 schematically illustrates the fuel cell power plant of FIG. 1operating in a second mode.

DETAILED DESCRIPTION

Embodiments of this invention provide a fuel cell power plant with theability to generate reactive power as the only output to support a localgrid system, for example.

FIG. 1 schematically illustrates selected portions of a fuel cell powerplant 20. A cell stack assembly (CSA) 22 includes a plurality of fuelcells (not specifically illustrated) that generate electrical powerbased on an electrochemical reaction. The fuel cells may take a varietyof forms. For example, some fuel cells will be phosphoric acid fuelcells while others will be polymer electrolyte membrane fuel cells.Those skilled in the art who have the benefit of this description willbe able to select an appropriate type of fuel cell and CSA arrangementto meet their particular needs.

In one example implementation, the fuel cell power plant 20 is a lowvoltage system because it provides a real power output that is less than600 kilowatts. Example implementations include a real power output of480 kilowatts or 440 kilowatts.

A plurality of inverters 24 are included for converting DC electricalpower from the CSA 22 into real AC power to be provided to a loadexternal to the fuel cell power plant 20. The illustrated exampleincludes at least one switch 26 for selectively coupling a selectednumber of the inverters 24 with the CSA 22 over a DC bus 28. Acontroller 30 controls operation of the switch 26 and the inverters 24to achieve a desired operation and output from the fuel cell power plant20. The controller 30 also controls a switch 32 that selectively couplesan AC bus 34 associated with the inverters 24 to an output 36 of thefuel cell power plant 20.

The fuel cell power plant 20 also includes a capacitor 38 and aplurality of loads associated with the operation of the fuel cell powerplant schematically shown at 40. Example loads included in the schematicrepresentation at 40 include pumps for circulating coolant or reactantsand blowers associated with the fuel cell power plant 20.

In FIG. 1, the controller 30 controls operation of the inverters 24according to a first mode of operation. In this example, the first modecorresponds to a real power mode of the fuel cell power plant 20. Whenit is desired to provide real AC power at the output 36, the controller30 operates the switches 26 and 32 to selectively couple the cell stackassembly 22 with one or more of the inverters 24 to provide real ACpower output at 36. The controller 30 in this example is programmed tocontrol the inverters using known techniques for providing such power onthe output 36. Limited reactive power may be provided in the first mode.

As schematically represented by the dashed lines in FIG. 1, thecapacitor 38 is not involved in providing real AC power output at 36.There are times, however, during the first mode of operation when atransient load is experienced by the fuel cell power plant 20. This mayoccur, for example, when there is a relatively sudden increase in thepower demand associated with the load external to the fuel cell powerplant 20 or when there is a drop in available power from an externalgrid associated with the power plant 20. Under such conditions, thecontroller 30 controls the switch 26 to provide load step transitionassistance using power from the capacitor 38. During the first mode ofoperation the capacitor 38 provides transient load support.

FIG. 2 schematically illustrates the fuel cell power plant 20 operatingin a second mode in which the output at 36 is exclusively reactivepower. In the second mode, the controller 30 controls the switches 26and 32 and the inverters 24 so that at least one of the inverters 24 andthe capacitor 38 provide the reactive power output at 36. The CSA 22 isnot used for providing any electrical output external to the fuel cellpower plant 20 in the second mode. As schematically represented by thedashed lines in FIG. 2, output from the CSA 22 provides power to theloads internal to the fuel cell power plant at 40 during the second modeof operation. In one example, there are seven inverters 24 with a firstone of them used in the second mode for providing power from the CSA 22to the loads at 40. The other six inverters 24 are utilized inassociation with the capacitor 38 for providing reactive power to orabsorbing reactive power from a grid external to the fuel cell powerplant 20.

The controller 30 controls operation of the inverters 24 associated withthe capacitor 38 in the second mode to provide reactive power output toor to absorb reactive power from a grid external to the fuel cell powerplant 20. Under both conditions, the capacitor 38 voltage is controlledby changing the phase angle of the inverters.

In the second mode, the portion of the DC bus 28 associated with the CSA22 is isolated from the portion of the DC bus 28 associated with thecapacitor 38. The controller 30 operates the switch 26 to realize the DCbus isolation. The AC bus 34 is similarly divided into a portion that isoperative for providing grid output at 36 and another portion forproviding power to the loads at 40. The controller 30 operates theswitch 32 to realize such a division of the AC bus 34 to isolate the ACoutput based on the CSA 22 from the reactive power output based on thecapacitor 38.

The inclusion of the capacitor 38 and the control of the inverters 24and switches 26 and 32 allows for the fuel cell power plant 20 tooperate in a first, real power mode and in a second, “reactive poweronly” mode where the output at 36 from the fuel cell power plant 20 isexclusively reactive power.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthe essence of this invention. The scope of legal protection given tothis invention can only be determined by studying the following claims.

We claim:
 1. A fuel cell power plant, comprising: a cell stack assemblyincluding a plurality of fuel cells configured to generate electricitybased on an electrochemical reaction; a capacitor; a plurality ofinverters; and at least one controller that is configured to control theplurality of inverters in a first mode and a second mode, the first modeincluding the cell stack assembly associated with at least one of theinverters, the cell stack assembly and the at least one of the invertersproviding real power to a load external to the fuel cell power plant,the second mode including at least a second one of the invertersassociated with the capacitor, the second one of the inverters and thecapacitor selectively providing reactive power to or receiving reactivepower from a grid external to the fuel cell power plant.
 2. The fuelcell power plant of claim 1, wherein the reactive power of the secondmode is the only power output from the fuel cell power plant of thesecond mode that is external to the fuel cell power plant.
 3. The fuelcell power plant of claim 1, wherein more than one of the inverters isassociated with the capacitor in the second mode.
 4. The fuel cell powerplant of claim 1, wherein the capacitor is charged by the reactive powerreceived from the grid in the second mode.
 5. The fuel cell power plantof claim 1, wherein the first mode includes the capacitor associatedwith at least one of the inverters; and the first mode includes thecapacitor providing supplemental power output from the fuel cell powerplant when there is an increase in a load demand on the fuel cell powerplant.
 6. The fuel cell power plant of claim 1, wherein the real poweris a low voltage power less than or equal to 600 kilowatts.
 7. The fuelcell power plant of claim 1, comprising a DC bus between the cell stackassembly and the plurality of inverters; and a switching device that iscontrolled by the controller to selectively couple the cell stackassembly, the capacitor, and selected ones of the plurality of invertersrespectively to the DC bus.
 8. The fuel cell power plant of claim 8,comprising an output configured to be coupled to a load external to thefuel cell power plant; an AC bus between the plurality of inverters andthe output; and at least one switching device that is controlled by thecontroller to selectively couple selected ones of the inverters to theoutput.
 9. The fuel cell power plant of claim 1, wherein the capacitoris the only source of power supplied by the fuel cell power plantexternal to the fuel cell power plant in the second mode.
 10. The fuelcell power plant of claim 1, wherein the second mode includes at least afirst one of the inverters associated with the cell stack assembly, thefirst one of the inverters and the cell stack assembly providing powerto at least one other component of the fuel cell power plant.
 11. Amethod of operating a fuel cell power plant including a cell stackassembly, a capacitor, and a plurality of inverters, the methodcomprising: controlling the plurality of inverters in a first mode and asecond mode; using the cell stack assembly and at least one associatedone of the inverters for providing real power to a load external to thefuel cell power plant in the first mode; and using the capacitor and atleast a second one of the inverters for selectively providing reactivepower to or receiving reactive power from a grid external to the fuelcell power plant in the second mode.
 12. The method of claim 11, whereinthe reactive power of the second mode is the only power output from thefuel cell power plant in the second mode that is external to the fuelcell power plant.
 13. The method of claim 11, wherein more than one ofthe inverters is associated with the capacitor in the second mode. 14.The method of claim 11, comprising charging the capacitor using thereactive power received from the grid in the second mode.
 15. The methodof claim 11, comprising providing supplemental power output from thefuel cell power plant using the capacitor and at least one of theinverters associated with the capacitor in the first mode when there isan increase in a load demand on the fuel cell power plant.
 16. Themethod of claim 11, wherein the real power is a low voltage power lessthan or equal to 600 kilowatts.
 17. The method of claim 11, wherein thecapacitor is the only source of power supplied by the fuel cell powerplant external to the fuel cell power plant in the second mode.
 18. Themethod of claim 11, comprising using the cell stack assembly and atleast one associated one of the inverters for providing power to atleast one other component of the fuel cell power plant in the secondmode.